The present invention generally relates to a superabrasive wheel, and more specifically, it relates to a superabrasive wheel for mirror finishing employed for mirror-finishing a hard brittle material such as silicon, glass, ceramics, ferrite, rock crystal, cemented carbide or the like.
Recently, high-precision mirror finishing of a material is required following abrupt technical innovation such as high integration of a semiconductor device or ultraprecision in working of ceramics, glass, ferrite or the like. Such mirror finishing is generally performed by grinding referred to as lapping. More specifically, free abrasive grains mixed into a lapping solution are fed between a lapping surface plate and a workpiece and rubbed with each other while applying pressure to the lapping surface plate and the workpiece in this grinding, for grinding the workpiece due to rolling and scratching actions of the free abrasive grains and providing a highly precise mirror-finished surface on the workpiece. In this lapping, however, a large quantity of free abrasive grains are consumed to result in a large quantity of mixture, referred to as sludge, of used freed abrasive grains, chips caused by cutting the workpiece and the lapping solution, disadvantageously leading to deterioration of the working environment and pollution.
Therefore, mirror finishing employing fixed fine superabrasive grains is actively studied/developed as a method substitutable for the aforementioned grinding employing free abrasive grains. As such mirror finishing employing fixed fine superabrasive grains, well known is machining with a resin bond superabrasive wheel elastically holding superabrasive grains of several xcexcm in mean grain size or ELID (electrolytic in-progress dressing) grinding of dressing a metal bond superabrasive wheel while electrolytically dissolving a bond material for grinding a material with the metal bond superabrasive wheel.
In the aforementioned machining employing a resin bond superabrasive wheel, however, the sharpness of a grindstone is deteriorated due to the fine superabrasive grains, and the grindstone is so remarkably worn that the worked surface of a workpiece is readily changed in shape or reduced in precision and the grindstone must be frequently trued and dressed.
In the aforementioned working method employing a metal bond superabrasive wheel, the rigidity of the metal bond material is so high that superabrasive grains finer than those in the resin bond superabrasive wheel must be used for obtaining a mirror-finished state substantially identical to the worked surface of the workpiece obtained by the machining employing the resin bond superabrasive wheel, to result in further deterioration of the sharpness of the grindstone.
In order to solve the problem of sharpness, a vitrified bond may be used as the binder while reducing the area of a superabrasive layer. For example, a number of grooves may be formed in a superabrasive layer employing a vitrified bond as the binder, so that superabrasive layers contributing to grinding are formed at intervals from each other. When employing a superabrasive wheel formed with such superabrasive layers, not only the conventional grinding employing free abrasive grains can be changed to grinding employing fixed superabrasive grains but also a vitrified bond superabrasive wheel for mirror finishing having remarkably excellent sharpness and a long life can be provided by performing truing and dressing with a diamond rotary dresser (hereinafter referred to as an RD). This is because large-volume pores of the vitrified bond serve as chip pockets for smoothly discharging chips and enabling highly efficient machining, so that the workpiece can be mirror-finished with small surface roughness.
In the aforementioned vitrified bond superabrasive wheel for mirror finishing, a plurality of segment superabrasive layers are arranged along the peripheral direction of an annular base plate at intervals from each other. Depending on the size or the shape of the segments, however, superabrasive grains crushed or falling during mirror finishing or shavings may be caught between the superabrasive layers and the workpiece, to cause scratches on the surface of the workpiece. Further, a long time is required for a step of removing such scratches.
For example, Japanese Patent No. 2976806 proposes a structure of a segment grindstone. This segment grindstone is formed with segment fixing grooves so that a plurality of abrasive layer segments are engaged in the segment fixing grooves respectively. When performing grinding with the segment grindstone having such a structure, however, the segment fixing grooves are clogged with shavings, and dischargeability for such shavings is extremely deteriorated.
Japanese Patent Laying-Open No. 54-137789 (1979) proposes a structure of a segment type grindstone for surface grinding. In the segment type grindstone disclosed in this gazette, superabrasive layers are formed by sintering superabrasive grains with a binder such as a metal bond or a resin bond. When arranging superabrasive layers of plate segments shown in FIG. 4 or FIG. 6 of this gazette along the peripheral direction of an annular base plate at intervals from each other, grinding resistance is disadvantageously increased due to the metal bond or the resin bond employed as the binder, although dischargeability for shavings is improved. Therefore, sharpness is deteriorated in grinding and the superabrasive layers are readily displaced from the base plate. The superabrasive layers are frequently displaced as the quantity of grinding is increased, to result in scratches. Consequently, the life of the grindstone is disadvantageously reduced.
The aforementioned gazette further proposes a structure of a segment type grindstone for surface grinding formed by arranging segment tips of cylindrically formed superabrasive layers along the peripheral direction of an annular base plate at intervals from each other in FIG. 1. However, although such cylindrical superabrasive layers are hardly displaced from the base plate in grinding, the inner sides of the cylindrical superabrasive layers are readily clogged with shavings and dischargeability for such shavings is disadvantageously deteriorated.
Accordingly, an object of the present invention is, in order to solve the aforementioned problems, to provide a superabrasive wheel for mirror finishing improved in dischargeability for superabrasive grains crushed or falling during mirror finishing or shavings to hardly cause scratches, capable of performing efficient machining and also capable of preventing scratches caused by displacement of a segment superabrasive layer by rendering the superabrasive layer hardly displaceable from a base plate.
According to a first aspect of the present invention, a superabrasive wheel for mirror finishing, comprising an annular base plate having an end surface and a plurality of superabrasive layers or members, each having a peripheral end surface, arranged along the peripheral direction of this annular base plate at intervals from each other and fixed onto the end surface of the base plate, has the following characteristics. Each of the plurality of superabrasive layers or members has a flat plate shape, and is so arranged that the peripheral end surface is substantially parallel to the rotary shaft of the superabrasive wheel. A surface defined by the thickness of the flat plate shape of each of the plurality of superabrasive layers, i.e., a surface along the direction of the thickness of the flat plate shape is fixed onto the end surface of the base plate. Superabrasive grains are bonded by a binder of a vitrified bond in the superabrasive layers.
In the superabrasive wheel having the aforementioned structure, the surface defined by the thickness is fixed onto the end surface of the base plate in each of the superabrasive layers having the flat plate shape, whereby sufficient clearances can be defined between the superabrasive layers and dischargeability for chips and shavings can be improved.
Further, the peripheral end surface of each superabrasive layer is arranged to be substantially parallel to the rotary shaft of the superabrasive wheel so that the position of a working surface of each superabrasive layer is kept substantially constant with respect to a workpiece in in-feed grinding although the superabrasive layer may be worn as the grinding progresses, whereby a stable working mode can be sustained. Therefore, the working surface of each superabrasive layer can be regularly brought into contact with the central portion of the workpiece. Thus, the finished surface of the workpiece is flattened.
In particular, the superabrasive grains are bonded by the binder of the vitrified bond in the flat-shaped superabrasive layers of the aforementioned superabrasive wheel, whereby grinding resistance can be reduced during grinding. Therefore, the superabrasive layers can be rendered hardly displaceable during grinding. Thus, the surface of the workpiece can be prevented from scratches resulting from displacement of the superabrasive layers.
Also when the quantity of working is increased, the grinding resistance can be kept low. Thus, reduction of the life resulting from displacement of the superabrasive layers can be prevented.
In the aforementioned superabrasive wheel for mirror finishing according to the first aspect, the superabrasive layers preferably have working surfaces substantially perpendicular to the rotary shaft of the superabrasive wheel, and the working area of the plurality of superabrasive layers preferably has a ratio of at least 5% and not more than 80% with respect to the area of a ring shape defined by a line connecting the outer peripheral edges of the plurality of superabrasive layers with each other and a line connecting the inner peripheral edges of the plurality of superabrasive layers with each other.
In the superabrasive wheel according to the present invention, each superabrasive layer is brought into the flat plate shape, thereby enabling control of reducing the area ratio of the working surface of the superabrasive layer and increasing the force acting on each superabrasive grain with respect to such continuous type superabrasive layers that integrated continuous superabrasive layers are formed on the end surface of the superabrasive wheel. Thus, grindability of the superabrasive wheel can be improved while an autogenous action of the superabrasive wheel can be smoothed. Assuming that the radial widths of the superabrasive layers having the flat plate shape are identical to each other, the area of the working surfaces of the plurality of superabrasive layers having a flat plate shape is preferably set to a ratio within the range of 5 to 80% of the area of the continuous type superabrasive layers, more preferably set within the range of 10 to 50%. Thus, working pressure of 2 to 10 times with respect to the continuous type superabrasive layers is applied to the working surface of each superabrasive layer of the flat plate shape in the superabrasive wheel according to the present invention, and a state of excellent sharpness can be sustained.
In the superabrasive wheel for mirror finishing according to the first aspect of the present invention, the superabrasive layers preferably contain superabrasive grains of at least 0.1 xcexcm and not more than 100 xcexcm in mean grain size. Synthetic superabrasive grains for a resin bond are suitable as the contained superabrasive grains. The synthetic superabrasive grains for a resin bond, having higher crushability as compared with synthetic superabrasive grains for a metal bond or a saw blade, are particularly preferable since small inserts can be formed on the forward ends of the superabrasive grains by truing and dressing with an RD.
As synthetic diamond superabrasive grains for a resin bond, RVM or RJK1 (trade name) by GE Superabrasives, IRM (trade name) by Tomei Diamond Kabushiki Kaisha or CDA (trade name) by De Beers can be applied. As the synthetic diamond superabrasive grains for a resin bond, BMP1 (trade name) by GE Superabrasives or SBNB, SBNT or SBNF (trade name) by Showa Denko K.K. can be applied.
While an RD is most preferably employed for performing truing and dressing in consideration of efficiency and molding precision, it is also possible to employ a metal bond grindstone or an electrodeposition grindstone having a diamond grain size of about #30 (grain diameter: 650 xcexcm) with no dispersion in forward end height of diamond abrasive grains.
According to a second aspect of the present invention, a superabrasive wheel for mirror finishing comprising an annular base plate having an end surface and a plurality of superabrasive layers or members, each having a peripheral end surface, arranged along the peripheral direction of the annular base plate at intervals from each other and fixed onto the end surface of the base plate, has the following characteristics. Each of the plurality of superabrasive layers or members has an angularly or curvedly bent plate shape, e.g. a V-shaped bent plate configuration, or a C-shaped curved plate configuration, and is so arranged that the peripheral end surface is substantially parallel to the rotary shaft of the superabrasive wheel. A surface defined by the thickness of the plate shape of each of the plurality of superabrasive layers is fixed onto the end surface of the base plate.
In the superabrasive wheel having the aforementioned structure, the surface defined by the thickness of the plate shape of each of the superabrasive layers, i.e., the surface along the direction of the thickness of the plate shape is fixed onto the end surface of the base plate similarly to the aforementioned superabrasive wheel according to the first aspect, whereby sufficient clearances can be defined between the plurality of superabrasive layers so that dischargeability for shavings and chips can be improved.
Further, each of the superabrasive layers is so arranged that the peripheral end surface is substantially parallel to the rotary shaft of the superabrasive wheel similarly to the aforementioned superabrasive wheel according to the first aspect, whereby the position of a working surface of each superabrasive layer remains substantially constant with respect to a workpiece also when the superabrasive layer is worn as grinding progresses in in-feed grinding, so that a stable working mode can be sustained. Therefore, the working surface of the superabrasive layer can be regularly brought into contact with the central portion of the workpiece. Thus, the finished surface of the workpiece is flattened.
Particularly in the superabrasive wheel according to the second aspect of the present invention, each of the plurality of superabrasive layers has the angularly bent plate shape. The surface defined by the thickness of the angular plate shape is fixed onto the end surface of the base plate, i.e., the shape of the surface of the superabrasive layer fixed to the end surface of the base plate is angular, whereby each superabrasive layer is strengthened against resistance in the vertical direction and the rotational direction of the superabrasive wheel applied to the superabrasive layer in grinding, to be hardly displaced from the end surface of the base plate. Thus, the surface of the workpiece can be prevented from scratches resulting from displacement of the superabrasive layer.
In the superabrasive layers of the superabrasive wheel for mirror finishing according to the second aspect of the present invention, superabrasive grains are preferably bonded by a binder of a vitrified bond. The vitrified bond can reduce grinding resistance in grinding as the binder, and hence the superabrasive layers can be rendered more hardly displaceable from the end surface of the base plate. Thus, the surface of the workpiece can be more effectively prevented from scratches resulting from displacement of the superabrasive layers. Further, the vitrified bond, acting to smooth an autogenous action of the superabrasive wheel as the binder, contributes to sustainment of excellent sharpness.
In the superabrasive layers of the superabrasive wheel for mirror finishing according to the second aspect of the present invention, superabrasive grains are preferably bonded by a binder of a resin bond. The resin bond, acting to smooth the autogenous action of the superabrasive wheel as the binder similarly to the aforementioned vitrified bond, contributes to sustainment of excellent sharpness. Further, the resin bond having an elastic action as the binder effectively reduces the sizes of scratches formed on the surface of the workpiece during grinding, thereby reducing surface roughness of the workpiece.
In the superabrasive wheel for mirror finishing according to the second aspect of the present invention, each of the plurality of superabrasive layers is preferably so arranged that an angularly bent portion is located on the inner peripheral side of superabrasive wheel. An open part opposite to the angularly bent and closed part is located on the outer peripheral side of the superabrasive wheel due to this structure, whereby shavings and chips caused during grinding can be readily discharged from the open part. Thus, dischargeability for shavings can be improved.
Each of the plurality of superabrasive layers preferably has a plate shape bent in a V shape. When each superabrasive layer of the plate shape is bent in the V shape, the superabrasive layer is strengthened against resistance in the vertical direction and the rotational direction of the superabrasive wheel applied to each superabrasive layer during grinding, to be more hardly displaceable from the end surface of the base plate. Therefore, it is possible to prevent occurrence of scratches resulting from displacement of the superabrasive layer during grinding.
When each of the superabrasive layers has the plate shape bent in the V shape, the apical angle of the V shape is preferably at least 30xc2x0 and not more than 150xc2x0. The apical angle of the V shape is set to at least 30xc2x0, in order to efficiently discharge shavings and chips during grinding. Further, the apical angle of the V shape is set to not more than 150xc2x0, so that a grinding fluid can be efficiently fed to a ground surface of the workpiece and the superabrasive layers are hardly displaceable from the end surface of the base plate against resistance in grinding. In order to improve these effects, the apical angle of the V shape is more preferably set to at least 45xc2x0 and not more than 90xc2x0.
As to the size of each superabrasive layer having the plate shape bent in the V shape, the length of a single side of the V shape, the thickness of the plate shape forming the V shape and the height of the plate shape forming the V shape, i.e., the length along the direction of the rotary shaft of the superabrasive wheel are preferably set to 2 to 20 mm, 0.5 to 5 mm and 3 to 10 mm respectively. More preferably, the length of a single side forming the V shape, the thickness of the plate shape forming the V shape and the height of the plate shape forming the V shape are set to 3 to 15 mm, 1 to 3 mm and 3 to 10 mm respectively. Further, the superabrasive layers having the plate shape bent in the V shape are preferably fixed onto the end surface of the base plate along the peripheral direction of the annular base plate at intervals of 0.5 to 20 mm from each other, and the intervals are more preferably set to 1 to 10 mm. The intervals between the superabrasive layers are preferably properly decided in response to grinding conditions and the type of the workpiece.
In the superabrasive wheel for mirror finishing according to the second aspect of the present invention, each of the plurality of superabrasive layers preferably has a plate shape bent to have a curved surface. In other words, a corner portion preferably has a radius of curvature in the bent shape of the superabrasive layer. When each superabrasive layer has the plate shape bent to have a curved surface, the grinding fluid can be efficiently fed while shavings and chips can be effectively discharged similarly to the case of the plate shape bent in the V shape, and the superabrasive layer is hardly displaceable from the end surface of the base plate against resistance in grinding. Thus, scratches resulting from displacement of the superabrasive layer can be prevented in grinding. A semicylindrical shape obtained by halving a cylindrical shape, a U shape, a C shape or the like can be employed as the plate shape bent to have a curved surface.
In the superabrasive wheel for mirror finishing according to the second aspect of the present invention, the superabrasive layers preferably have working surfaces substantially perpendicular to the rotary shaft of the superabrasive wheel, and the working area of the plurality of superabrasive layers preferably has a ratio of at least 5% and not more than 80% with respect to the area of a ring shape defined by a line connecting the outer peripheral edges of the plurality of superabrasive layers with each other and a line connecting the inner peripheral edges of the plurality of superabrasive layers with each other.
The shape of each superabrasive layer is brought into the plate shape thereby enabling control of reducing the area ratio of the working surface of the superabrasive layer and increasing the force acting on each superabrasive grain with respect to such a continuous type superabrasive layer that a single integrated continuous superabrasive layer is formed on the end surface of the superabrasive wheel, improving grindability and smoothing the autogenous action of the superabrasive wheel. Assuming that the radial lengths of the superabrasive layers are identical to each other, the area of the working surfaces of the plurality of superabrasive layers is preferably set to 5 to 80% of the area of the continuous type superabrasive layer, more preferably set within the range of 10 to 50%. Thus, working pressure of 2 to 10 times with respect to the continuous type superabrasive layer is applied to the working surface of each superabrasive layer in the superabrasive wheel according to the present invention, and a state of excellent sharpness can be sustained.
In the superabrasive wheel for mirror finishing according to the second aspect of the present invention, the superabrasive layers preferably contain superabrasive grains of at least 0.1 xcexcm and not more than 100 xcexcm in mean grain size. When employing a vitrified bond or a resin bond as a binder for the superabrasive wheel according to the second aspect of the present invention, synthetic superabrasive grains for a resin bond are suitable as the contained superabrasive grains. The synthetic superabrasive grains for a resin bond, having higher crushability as compared with synthetic superabrasive grains for a metal bond or a saw blade, are particularly preferable since small inserts can be formed on the forward ends of the superabrasive grains by truing and dressing with an RD.
As synthetic diamond superabrasive grains for a resin bond, RVM or RJK1 (trade name) by GE Superabrasives, IRM (trade name) by Tomei Diamond Kabushiki Kaisha or CDA (trade name) by De Beers can be applied. As the synthetic diamond superabrasive grains for a resin bond, BMP1 (trade name) by GE Superabrasives or SBNB, SBNT or SBNF (trade name) by Showa Denko K.K. can be applied.
While an RD is most preferably employed for truing and dressing the superabrasive wheel according to the present invention in consideration of efficiency and molding precision, it is also possible to employ a metal bond grindstone or an electrodeposition grindstone having a diamond grain size of about #30 (grain diameter: 650 xcexcm) with no dispersion in forward end, height of diamond abrasive grains.
When employing the superabrasive wheel for mirror finishing according to the present invention for grinding, as hereinabove described, it is possible to effectively prevent superabrasive grains crushed or falling during grinding or shavings and chips from being caught between the superabrasive layers and the workpiece and causing scratches on the surface of the workpiece. Thus, dischargeability for superabrasive grains or shavings can be improved while the superabrasive layers are hardly displaceable from the end surface of the base plate during grinding, whereby scratches resulting from displacement of the superabrasive layers can also be prevented.